Paleoenvironment of the Eocene coal seam in the Fushun Basin (NE China): Implications from petrography and organic geochemistry

The Fushun Basin in northeast China is a strike-slip basin filled with coal-bearing Eocene sediments. The basin fill includes from base to top volcaniclastic rocks, a 20 to 145 m thick coal seam (Lower Eocene Guchengzi Formation) and lacustrine oil shale, up to 300 m thick (Jijuntun Formation). The sub-bituminous coal of the Guchengzi Formation, famous for the amber inclusions, is exploited in the largest opencast coal and oil shale mine in Asia. In the present contribution the depositional environment and the thermal history of the Guchengzi Formation are investigated based on macro- and micropetrographic data, bulk geochemical parameters, biomarker analysis, stable isotope geochemistry, and vitrinite reflectance measurements. According to our interpretation, accumulation of the low-sulfur coal commenced in a low-lying mire near a shallow lake. Subsequently low-ash coal was deposited either in a low-lying mire protected from clastic influx or in a raised mire. Finally, high subsidence rates, typical for strike-slip basins, caused the flooding of the mire and terminated peat accumulation. Pristane/phytane ratios suggest a gradual change from oxic to less oxic conditions during the drowning phase of the seam. Organic-rich shales accumulated in the newly formed freshwater lake and resulted in the deposition of the overlying oil shale-bearing Jijuntun Formation. The coal is rich in liptinite macerals, especially sporinite. Gymnosperms dominated the paleovegetation of the mire, something that is also reflected by the frequent occurrence of resinite and amber and their biomarker composition. The coal reached the sub-bituminous or even the high volatile bituminous stage, despite the limited burial depth. Simple 1D models suggest that this is a result of high heat flow, rather than of deep burial and erosion. The hydrogen-rich coal is oil-prone. (C) 2014 Elsevier B.V. All rights reserved.